As other people have said, plenty of baseline humans (IG heroes, SoB, inquisitors, etc) on tabletop have better stats than standard marines. Lorewise, even a basic human could outfight an Astartes. Ciaphas Cain himself kills at least one chaos space marine in a sword duel (CSMs, while being retarded, are still superhumans with thousands of years of fighting experience), and his assistent Jurgen kills one with a meltagun. There's also that hilarious meme of an Elysian kamikazing one with a grenade.
Astartes are just augmented humans in fancy armor. Rogue Traders and their retinue are super-wealthy, elite combatants that can afford the best gear available. The disparity between their fighting prowess is not the huge gap it may seem.
Likely the Space Wolf here will just be very tanky compared to the other party members.
When that happens, it's either because the human is augmented or we are seeing some shit writing (which Warhammer is full of). A Space Marine is capable of swinging a sword many times more powerful than a human could ever hope to block, and that's not even to mention the fact that they can hit speeds of thirty to forty five miles per hour in their fucking armor. Warhammer is filled with inconsistencies so there are plenty of examples of Space Marines being unstoppable to a ridiculous degree, as well as examples of them basically being slightly augmented humans in fancy armor as you said. When you look at the actual setting though and the way they "should" be, it doesn't make sense for them to not be significantly superior to human soldiers, while still being vulnerable in a sense.
Theoretically it could be possible to balance encounters around him, but I'd argue that would require so much focus that the game would essentially turn into balancing each encounter around one of your companions, which doesn't make sense.
